Thin-Film Transistor, Method for Manufacturing the Same and Display Device Comprising the Same

ABSTRACT

A thin-film transistor includes a substrate, a first gate electrode formed on the substrate, a first active layer that is formed on the substrate and includes a first oxide semiconductor layer and a first barrier layer, a second active layer that is formed on the first active layer and includes a second oxide semiconductor layer and an intermediate barrier layer, a gate insulating layer that is formed on the second active layer, a second gate electrode that is formed on the gate insulating layer and is electrically connected to the first gate electrode, an interlayer insulating film formed on the second gate electrode, the first active layer and the second active layer, and a source electrode and a drain electrode electrically connected to the first active layer and the second active layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application No.10-2013-0034761 filed on Mar. 29, 2013 and Korean Patent Application No.10-2013-0167911 filed on Dec. 30, 2013 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide semiconductor based thin-filmtransistor and a method for manufacturing the same, and moreparticularly to an oxide semiconductor based thin-film transistor withan intermediate barrier layer for improving reliability of the thin-filmtransistor.

2. Description of the Related Art

Recently, with growing interest in information displays and anincreasing demand for portable electronic devices, light and thin-filmtype flat panel display (FPD) devices have been widely studied andcommercialized. In particular, among the flat panel displays, liquidcrystal display (LCD) devices and organic light-emitting display (OLED)devices have been widely studied, and a thin-film transistor (TFT) hasbeen used as a switching element and/or a driving element in the LCDdevice and the OLED device.

The thin-film transistor is classified into a thin-film transistor usingamorphous-silicon, a thin-film transistor using poly-silicon and athin-film transistor using an oxide semiconductor according to materialsused as an active layer. When the thin-film transistor usingpoly-silicon is manufactured, a process for implanting ions to adjustresistance of the active layer is further performed, and an ionimplantation process using an additional mask for defining an ionimplantation region is further performed. For this reason, there is adisadvantage in process. The thin-film transistor using an oxidesemiconductor has higher mobility than that of the thin-film transistorusing amorphous-silicon semiconductor. Further, the thin-film transistorwith an oxide semiconductor generally exhibit lower leakage current thanthe thin-film transistor with amorphous-silicon semiconductor andpoly-silicon semiconductor, and reliability of the thin-film transistorusing an oxide semiconductor is relatively higher than those of thethin-film transistor using amorphous silicon and the thin-filmtransistor using poly-silicon. Furthermore, the thin-film transistorusing an oxide semiconductor has an advantage in that uniformdistribution characteristics of a threshold voltage are obtained ascompared to the thin-film transistor using poly-silicon.

During the operation of the oxide semiconductor based TFT, carriers havea tendency to be accumulated in the insulation layer and to remain“trapped” in the insulation layer throughout the operation of the TFT.Some of the trapped carriers remain in the insulation layer even afterthe TFT is turned off. In most instances, once carriers are trapped,they remain trapped throughout the on and off states of the TFT,possibly for the entire life of the TFT. This “trapping” of carrierscauses the threshold voltage to gradually shift, and the amount ofthreshold shift is generally correlated to the amount of deep trapdensity.

SUMMARY OF THE INVENTION

Embodiments relate to a thin-film transistor (TFT) including a firstoxide semiconductor layer and a second oxide semiconductor layer, anintermediate barrier separating the first and second oxide semiconductorlayers. A first gate insulating layer is disposed on a first gateelectrode. The first and second oxide semiconductor layers are disposedon the first gate insulation film. A second gate insulating layer isdisposed on the second oxide semiconductor layer. A second gateelectrode is disposed on the second gate insulating layer. The secondgate electrode is electrically connected to the first gate electrode. Asource electrode is electrically connected to the first and second oxidesemiconductor layers. A drain electrode is electrically connected to thefirst and second oxide semiconductor layers.

In one embodiment, a first current path is induced in the first oxidesemiconductor layer by applying a first voltage to the first gateelectrode and a second current path is induced in the second oxidesemiconductor layer by applying a second voltage to the second gateelectrode.

In one embodiment, the TFT is configured to receive the first and secondvoltages simultaneously. Also, in one embodiment, the first and secondvoltage s may be identical to each other.

In one embodiment, the TFT is an N-type TFT, and the intermediatebarrier layer includes a barrier material having a maximum valance bandvalue (V_(max)) lower than the material of at least one of the first andsecond oxide semiconductor layers. In one embodiment, the TFT is aP-type TFT, and the intermediate barrier layer includes a barriermaterial having a maximum conduction band value (C_(max)) greater thanthe material of at least one of the first and second oxide semiconductorlayers.

In one embodiment, the first oxide semiconductor layer has across-sectional width equal to or greater than that of the second oxidesemiconductor layer, and the source and drain electrodes are in directcontact with the first and second oxide semiconductor layers.

In one embodiment, a first barrier layer is interposed between the firstgate insulating layer and the first oxide semiconductor layer. A secondbarrier layer is interposed between the second gate insulating layer andthe second oxide semiconductor layer.

In one embodiment, the TFT is an N-Type TFT, and the first barrier layerincludes a barrier material having a maximum valance band value(V_(max)) lower than that of at least one of the first oxidesemiconductor layer and the first gate insulating layer. In oneembodiment, the TFT is a P-Type TFT, and the first barrier layerincludes a barrier material having a maximum conduction band value(C_(max)) greater than that of at least one of first oxide semiconductorlayer and the first gate insulating layer.

In one embodiment, the TFT is an N-Type TFT, and the second barrierlayer includes a barrier material having a maximum valance band value(V_(max)) lower than that of at least one of the second oxidesemiconductor layer and the second gate insulating layer. In oneembodiment, the TFT is an P-Type TFT, and the second barrier layerincludes a barrier material having a maximum conduction band value(C_(max)) greater than that of at least one of the second oxidesemiconductor layer and the second gate insulating layer.

In one embodiment, the first gate electrode is made of a reflectiveconductive material.

In one embodiment, the TFT is an N-Type TFT, and the intermediatebarrier layer includes at least one of TiOx, TaOx, SrTiO₃, BaZrO₃, ZrO₂,HfO₂, Al₂O₃, MgO and Ga₂O₃. In one embodiment, the TFT is a P-Type TFT,and the intermediate barrier layer includes at least one of Cu₂O,CuAlO₂, SiO₂, SrCu₂O₂ and Al₂O₃.

Embodiments also relate to a thin-film transistor (TFT) including afirst barrier layer interposed between an oxide semiconductor layer anda first gate insulating layer. In one embodiment, the TFT is an N-TypeTFT, and the first barrier layer includes a barrier material having amaximum valance band value (V_(max)) lower than that of at least one ofthe first oxide semiconductor layer and the first gate insulating layer.In one embodiment, the TFT is a P-Type TFT, and the first barrier layerincludes a barrier material having a maximum conduction band value(C_(max)) greater than that of at least one of first oxide semiconductorlayer and the first gate insulating layer.

The TFT may further include a second barrier layer interposed betweenthe oxide semiconductor layer and a second gate insulating layer. In oneembodiment, the TFT is an N-Type TFT, and the second barrier layercomprises a second material having a maximum valance band value(V_(max)) lower than that of the oxide semiconductor layer and thesecond gate insulating layer. In one embodiment, the TFT is a P-TypeTFT, and the second barrier layer comprises a second material having amaximum conduction band value (C_(max)) greater than that of the oxidesemiconductor layer and the second gate insulating layer.

In one embodiment, the oxide semiconductor layer includes indium (In),gallium (Ga) and Zinc (Zn).

Embodiments also relate to a method for manufacturing a thin-filmtransistor (TFT). A first gate electrode is formed on a substrate. Afirst gate insulating layer is formed on the first gate electrode. Anoxide semiconductor layer and a barrier layer are formed on the firstgate insulating layer.

In one embodiment, the barrier layer is formed between the first gateinsulating layer and the oxide semiconductor layer. When the TFT is anN-Type TFT, the barrier layer includes a barrier material having amaximum valance band value (V_(max)) lower than that of the oxidesemiconductor layer and the first gate insulating layer. When the TFT isa P-Type TFT, the barrier layer includes a barrier material having amaximum conduction band value (C_(max)) greater than that of the oxidesemiconductor layer and the first gate insulating layer.

In one embodiment, a second gate insulating layer and a second gateelectrode are formed on the oxide semiconductor layer such that thesecond gate insulating layer is formed between the oxide semiconductorlayer and the second gate electrode. The barrier layer is formed betweenthe second gate insulating layer and the oxide semiconductor layer. Whenthe TFT is an N-Type TFT, the barrier layer includes a barrier materialhaving a maximum valance band value (V_(max)) lower than that of theoxide semiconductor layer and the first gate insulating layer. When theTFT is a P-Type TFT, the barrier layer includes a barrier materialhaving a maximum conduction band value (C_(max)) greater than that ofthe oxide semiconductor layer and the first gate insulating layer.

In one embodiment, the barrier layer is formed between the first gateinsulating layer and the oxide semiconductor layer, and another barrierlayer is formed between the second gate insulating layer and the oxidesemiconductor layer.

In one embodiment, the oxide semiconductor layer is formed of a firstoxide semiconductor layer and a second oxide semiconductor layer, andthe barrier layer is formed between the first and second oxidesemiconductor layers. When the TFT is an N-Type TFT, the barrier layerincludes a barrier material having a maximum valance band value(V_(max)) lower than that of the first and second oxide semiconductorlayers. When the TFT is a P-Type TFT, the barrier layer includes abarrier material having a maximum conduction band value (C_(max))greater than that of the first and second oxide semiconductor layers.

In one embodiment, the barrier layer is formed between the first gateinsulating layer and the first oxide semiconductor layer. Anotherbarrier layer is formed on the first oxide semiconductor layer, and asecond oxide semiconductor layer is formed on the other barrier layer.Yet another barrier layer is formed on the second oxide semiconductorlayer, and then a second gate insulating layer is formed on the secondoxide semiconductor. A second gate electrode is formed on the secondgate insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1A is a plan view for describing a thin-film transistor accordingto an exemplary embodiment of the present invention;

FIG. 1B is a cross-sectional view of the thin-film transistor takenalong line Ib-Ib′ of FIG. 1A;

FIGS. 1C and 1D are energy band diagrams for describing the function ofintermediate barrier layer of the thin-film transistor according to theexemplary embodiment of the present invention;

FIGS. 1E to 1I are cross-sectional views of thin-film transistorsaccording to various exemplary embodiments of the present invention;

FIGS. 2A and 2B are cross-sectional views of thin-film transistorsaccording to various exemplary embodiments of the present invention;

FIG. 3A is a conceptual diagram of a display device according to anexemplary embodiment of the present invention;

FIG. 3B is an enlarged conceptual diagram of a sub-pixel region shown inFIG. 3A;

FIG. 3C is a plan view for describing the display device according tothe exemplary embodiment of the present invention;

FIG. 3D is a cross-sectional view taken along lines IIId-IIId′ andIIId″-IIId′″ of FIG. 3C;

FIG. 4 is a flowchart for describing a method for manufacturing athin-film transistor according to an exemplary embodiment of the presentinvention; and

FIGS. 5A to 5D are cross-sectional views of processes for describing themethod for manufacturing a thin-film transistor according to theexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Various advantages and features of the present invention and methodsaccomplishing thereof will become apparent from the followingdescription of embodiments with reference to the accompanying drawings.However, the present invention is not limited to exemplary embodimentdisclosed herein but will be implemented in various forms. The exemplaryembodiments are provided by way of example only so that a person ofordinary skilled in the art can fully understand the disclosures of thepresent invention and the scope of the present invention. Therefore, thepresent invention will be defined only by the scope of the appendedclaims.

Indicating that elements or layers are “on” other elements or layersinclude both a case in which the corresponding elements are just aboveother elements and a case in which the corresponding elements areintervened with other layers or elements.

Although first, second, and the like are used in order to describevarious components, the components are not limited by the terms. Theabove terms are used only to discriminate one component from the othercomponent. Therefore, a first component mentioned below may be a secondcomponent within the technical spirit of the present invention.

The same reference numerals indicate the same elements throughout thespecification.

In the drawings, size and thickness of each element are arbitrarilyillustrated for convenience of description, and the present invention isnot necessarily limited to those illustrated in the drawings.

In this specification, a flexible display device means an organic lightemitting display device to which flexibility is granted, and may be usedas the same meaning as a bendable display device, a rollable displaydevice, an unbreakable display device, a foldable display device, andthe like. In this specification, the flexible organic light emittingdisplay device is one example of various flexible display devices.

In this specification, a transparent display device means a displaydevice in which at least a partial area in a screen of the displaydevice viewed by a viewer is transparent. In this specification, thetransparent display device means a display device which is transparentin which transparency of the transparent display device is at least alevel a display device which is transparent for a user to recognize anobject behind the display device. The transparent display device in thepresent specification includes a display region and a non-displayregion. The display region is a region where a video or the like isdisplayed, and the non-display region is a region, such as bezel, wherethe video is not displayed. In order to maximize transmittance of thedisplay region, in the transparent display device, non-transparentelements such as a battery, a PCB (Printed Circuit Board), and a metalframe are arranged not under the display region but under thenon-display region. In this specification, the transparent displaydevice means, for example, a display device in which transmittance ofthe transparent display device is at least 20% or more. Thetransmittance in the present specification means a value obtained bydividing the amount of light transmitted through the transparent displaydevice by the total amount of entered light except for light which hasentered a transmitting region of the transparent display device and hasbeen reflected from an interface of the respective layers of thetransparent display device.

A front surface and a rear surface of the transparent display device inthe present specification are defined in view of light emitted from thetransparent display device. In the present specification, the frontsurface of the transparent display device means a surface on which lightis emitted from the transparent display device, and the rear surface ofthe transparent display device means a surface opposite to the surfaceon which light is emitted from the transparent display device.

Respective features of various exemplary embodiments of the presentinvention can be partially or totally joined or combined with each otherand as sufficiently appreciated by those skilled in the art, variousinterworking or driving can be technologically achieved and therespective exemplary embodiments may be executed independently from eachother or together executed through an association relationship.

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings.

Oxide semiconductors are becoming more popular for use as active layersin TFT due to its high mobility and other beneficial characteristics.However, the oxide semiconductors are susceptible to degradation byexposure to light and other factors such as the fabrication process ofthe TFT. The threshold voltage of the TFT may change as the result ofthe fabrication process or, even worse, change over time and operationsof the TFT as the TFT is exposed to light. It is desirable that thethreshold voltage of the TFT remains constant to provide a consistentoperation of the TFT. Embodiments relate to providing one or morebarrier layers to prevent degradation of the oxide semiconductors in theTFT as a result of the fabrication process and/or exposure to the light.

FIG. 1A is a plan view for describing a thin-film transistor accordingto an exemplary embodiment of the present invention. FIG. 1B is across-sectional view of the thin-film transistor taken along line Ib-Ib′of FIG. 1A. Referring to FIGS. 1A and 1B, a thin-film transistor 100Aincludes a substrate 110A, a first gate insulating layer 163A, a firstgate electrode 121A, a first active layer 140A, a second active layer150A, a second gate insulating layer 161A, a second gate electrode 122A,an interlayer insulating film 162A, a source electrode 131A, and a drainelectrode 132A.

The substrate 110A is a member for supporting various elements that canbe formed on the substrate 110A. The substrate 110A may be made from aninsulating material such as glass or plastic, but is not limitedthereto. The substrate may be made from various materials.

The substrate 110A may be made from various materials depending onvarious applications in which the thin-film transistor 100A is used. Forexample, when the thin-film transistor 100A is used in a flexibledisplay device, the substrate 110A may be made from a flexibleinsulating material. Here, examples of the flexible insulating materialinclude polyimide (PI), polyetherimide (PEI), polyethyeleneterephthalate (PET), polycarbonate (PC), polystyrene (PS),styrene-acrylonitrile copolymer (SAN), and silicone-acrylic resin.Further, when the thin-film transistor 100A is used in a transparentdisplay device, the substrate 110A may be made from a transparentinsulating material. Although it has been described that the thin-filmtransistor 100A includes the substrate 110A, the substrate 110A may bediscrete element separate from the thin-film transistor 100A. In suchcases, the substrate is an independent element separated from thethin-film transistor serving as a base for supporting the elementsincluded in the thin-film transistor.

The first gate electrode 121A as a lower gate electrode is formed on thesubstrate 110A. The first gate electrode 121A transmits a driving signalto the thin-film transistor 100A. The first gate electrode 121A isoverlapped with the first active layer 140A, specifically, a first oxidesemiconductor layer 141A of the first active layer 140A.

The first gate electrode 121A may be made from a conductive metalmaterial. The first gate electrode 121A may be made from any one of, forexample, molybdenum (Mo), aluminum (Al), chrome (Cr), gold (Au),titanium (Ti), nickel (Ni), neodymium (Nd) and copper (Cu), or an alloythereof, but is not limited thereto. The first gate electrode 121A maybe made from various materials. Moreover, the first gate electrode 121Amay be multiple layers made from any one selected from a groupconsisting of molybdenum (Mo), aluminum (Al), chrome (Cr), gold (Au),titanium (Ti), nickel (Ni), neodymium (Nd) and copper (Cu), or an alloythereof.

The first gate insulating layer 163A is formed on the substrate 110A.The first gate insulating layer 163A is formed to cover the first gateelectrode 121A on the substrate 110A. The first gate insulating layer163A prevents moisture or other impurities from permeating through thesubstrate 110A. The first gate insulating layer 163A is made from aninsulating film. A material constituting the first gate insulating layer163A may be selected depending on a kind of the substrate 110A or a kindof the thin-film transistor 100A. For example, the first gate insulatinglayer 163A may be a silicon oxide film, a silicon nitride film, ormultiple layers including the silicon oxide film and the silicon nitridefilm.

The first active layer 140A, as a lower active layer, is formed on thefirst gate insulating layer 163A. The first active layer 140A is formedabove the first gate electrode 121A to serve as the lower active layerof the thin-film transistor 100A and provides a conductive channel(path) at a lower side of the TFT (hereinafter referred to as “the lowerchannel”). The first active layer 140A includes a first oxidesemiconductor layer 141A that is formed above the first gate insulatinglayer 163A to provide the lower channel and a first barrier layer 142Athat is formed between the first gate insulating layer 163A and thefirst oxide semiconductor layer 141A to reduce degradation of thethin-film transistor 100A by light.

The first oxide semiconductor layer 141A is formed on the first barrierlayer 142A. The first oxide semiconductor layer 141A is formed to havesubstantially the same area as that of the first barrier layer 142A onthe first barrier layer 142A. More specifically, the first oxidesemiconductor layer 141A and the first barrier layer 142A are positionedto overlap with each other. In some other embodiments, however, thefirst oxide semiconductor layer 141A and the first barrier layer 142 maybe offset in horizontal locations and may have different horizontaldimensions.

The first oxide semiconductor layer 141A may be made from various metaloxides. Examples of a constituent material of the first oxidesemiconductor layer 141A includes a quaternary metal oxide such as anindium-tin-gallium-zinc-oxide (In—Sn—Ga—Zn—O)-based material, a ternarymetal oxide such as an indium-gallium-zinc-oxide (In—Ga—Zn—O)-basedmaterial, an indium-tin-zinc-oxide (In—Sn—Zn—O)-based material, anindium-aluminum-zinc-oxide (In—Al—Zn—O)-based material, anindium-hafnium-zinc-oxide (In—Hf—Zn—O)-based material, atin-gallium-zinc-oxide (Sn—Ga—Zn—O)-based material, analuminum-gallium-zinc-oxide (Al—Ga—Zn—O-based material) and atin-aluminum-zinc-oxide (Sn—Al—Zn—O)-based material, and a binary metaloxide such as an indium-zinc-oxide (In—Zn—O)-based material, atin-aluminum-zinc-oxide (Sn—Zn—O)-based material, an aluminum-zinc-oxide(Al—Zn—O)-based material, a zinc-magnesium-oxide (Zn—Mg—O)-basedmaterial, a tin-magnesium-oxide (Sn—Mg—O)-based material, anindium-magnesium-oxide (In—Mg—O)-based material, an indium-gallium-oxide(In—Ga—O)-based material, an indium-oxide (In—O)-based material, atin-oxide (Sn—O)-based material and a zinc-oxide (Zn—O)-based material.Composition ratios of the elements included in the respective oxidesemiconductor materials are not particularly limited, and may beadjusted at various composition ratios.

The first oxide semiconductor layer 141A overlaps with the first gateelectrode 121A, comes in contact with the source electrode 131A and thedrain electrode 132A, and provides the lower channel when a gate voltageis applied to the first gate electrode 121A. The first barrier layer142A is formed between the first gate insulating layer 163A and thefirst oxide semiconductor layer 141A. The first barrier layer 142A is alayer for reducing the degradation of the thin-film transistor 100A bylight, and specifically, is a layer for suppressing hole conductionbetween the first gate insulating layer 163A and the first oxidesemiconductor layer 141A. The first barrier layer 142A will be explainedin more detail with reference to FIG. 1C.

FIG. 1C is an energy band diagram for describing the thin-filmtransistor according to the exemplary embodiment of the presentinvention. For the sake of convenience in description, in FIG. 1C, onlythe first gate insulating layer 163A, the first barrier layer 142A andthe first oxide semiconductor layer 141A of the thin-film transistor100A are illustrated. Moreover, FIG. 1C illustrates a case the thin-filmtransistor is an n-type transistor as an example.

The first barrier layer 142A is the layer for reducing the degradationof the thin-film transistor 100A by light and serves as a chargetrapping barrier for reducing the degradation of the thin-filmtransistor 100A due to exposure to light. The first barrier layer 142Ais formed of a material having a maximum valence band value, which islower than that of the material forming the first gate insulating layer163A. The function of the first barrier layer 142A in terms of an energyband gap, a valence band and a conduction band is described herein withreference to FIGS. 1C and 1D.

The valence band is an energy band that is formed by the interaction oforbitals inside atoms, and refers to an energy band within whichelectrons having continuous energies do not move to other atoms whilethe electrons are bound to individual atoms. The conduction band is anenergy band that is formed by the overlap of orbitals outside atoms, andrefers to an energy band in which electrons having continuous energiescan freely move from their atoms to other atoms. The electronspositioned within the conduction band are called free electrons. Anenergy band gap “Eg” means the difference in energy values between thevalence band and the conduction band, more specifically, the differencebetween a maximum energy value “Vmax” of the valence band, which is arelatively lower energy band and a minimum energy value “Cmin” of theconduction band, which is a relatively higher energy band. In general,as an energy band gap of a material is low, the material is a conductor,and as an energy band gap of a material is high, the material is aninsulator.

In the n-type thin-film transistor 100A, when light illuminates thefirst oxide semiconductor layer 141A while the gate voltage is appliedto the first gate electrode 121 a, hole/electron pairs may be generatedin the first oxide semiconductor layer 141A. Holes “h” may be trapped inthe first gate insulating layer 163A during the operation of thethin-film transistor, leaving electrons “e” in the first oxidesemiconductor layer 141A. Some of the trapped carriers remain in theinsulation layer even after the TFT is turned off. Since, the number ofelectrons generated in the first oxide semiconductor layer 141A dependson the number of the holes “h” at the interface of the first oxidesemiconductor layer 141A and the first gate insulating layer 163A, theholes trapped in the first gate insulating layer 163A can graduallyshift the threshold voltage Vth of the thin-film transistor.

Accordingly, the first barrier layer 142A is used for reducing shiftingof the threshold voltage Vth of the thin-film transistor 100A.Specifically, the first barrier layer 142A has a lower maximum valenceband value (Vmax) than the maximum valence band value of the first gateinsulating layer 163A that is in contact with the first barrier layer142A. The maximum valence band value of the first barrier layer 142A isalso lower than the maximum valence band value of the first oxidesemiconductor layer 141A. As such, the material of the first barrierlayer 142A is such that the difference between the maximum valence bandvalue of the first gate insulating layer 163A and the maximum valenceband of the first barrier layer 142A is larger than the differencebetween the maximum valence band value of the first oxide semiconductorlayer 141A and the maximum valence band value of the first barrier layer142A.

With a maximum valence band value that is lower than the maximum valenceband value of the first oxide semiconductor layer 141A and the firstgate insulating layer 163A, the first barrier layer 142A makes itdifficult for the holes generated in the first oxide semiconductor layer141A to pass through the first barrier layer 142A and reach the firstgate insulating layer 163A. This reduces the amount of holes beingtrapped in the first gate insulating layer 163A, thereby reducing thethreshold voltage (Vth) shift of the thin-film transistor 100A.

As described above, the functionality of the first barrier layer 142Adepends on the relative difference of the maximum valence band value ofthe adjacent layers. As such, the material for forming the first barrierlayer 142A can vary depending on the material of the first oxidesemiconductor layer 141A and the first gate insulating layer 163A incontact with the first barrier layer 142A. In way of an example, thefirst oxide semiconductor layer 141A may be formed ofindium-gallium-zinc oxide (IGZO), and the first gate insulating layer163A may be formed of silicon nitride. In this example, the firstbarrier layer 142A may be formed of any one of barium zirconate(BaZrO₃), zirconium dioxide (ZrO₂), magnesium oxide (MgO), gallium oxide(Ga₂O₃), strontium titanate oxide (SrTiO₃), tantalum oxide (TaOx),aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), hafnium silicon oxide(HfSiO₄), yttrium oxide (Y₂O₃), and titanium oxide (TiOx), and acombination thereof. For example, the first barrier layer 142A formed ofTaOx exhibits superior hole blocking ability, making it an excellentbarrier material for preventing the hole trapping in the first gateinsulating layer 163A.

FIG. 1D is an energy band diagram for describing the thin-filmtransistor according to the exemplary embodiment in which the thin-filmtransistor is a p-type transistor.

In the p-type thin-film transistor 100A, when light illuminates thefirst oxide semiconductor layer 141A while the gate voltage is appliedto the first gate electrode 121A, electrons may be generated in thefirst oxide semiconductor layer 141A, and the generated electrons may betrapped in the first gate insulating layer 163A. Holes may be generatedin the first oxide semiconductor layer 141A by the number of the trappedelectrons, and the threshold voltage of the thin-film transistor 100Amay be shifted to cause the deterioration in characteristics of thethin-film transistor 100A.

The first barrier layer 142A is an element for reducing the shifting ofthe threshold voltage Vth of the thin-film transistor 100A. Theconstituent material of the first barrier layer 142A is determined suchthat a difference between the maximum energy value of the conductionband of the material constituting the first gate insulating layer 163Aand the maximum energy value of the conduction band of the materialconstituting the first barrier layer 142A is larger than a differencebetween the maximum energy value of the conduction band of the materialconstituting the first oxide semiconductor layer 141A and the maximumenergy value of the conduction band of the material constituting thefirst barrier layer 142A. Specifically, the first barrier layer 142A ismade from a material having a maximum energy value of a conduction bandwhich is greater than a maximum energy value of a conduction band of thematerial constituting the first gate insulating layer 163A coming incontact with the first barrier layer 142A and is greater than a maximumenergy value of a conduction band of the material constituting the firstoxide semiconductor layer 141A. When the first barrier layer 142A isformed so as to have the aforementioned maximum energy value relation,it is difficult for the electrons to pass through the first barrierlayer 142A and be trapped in the first gate insulating layer 163A. Assuch, the first barrier layer 142A can reduce the deterioration incharacteristics of the thin-film transistor 100A that is likely to occurdue to the shifted threshold voltage of the thin-film transistor 100A.

Similar to the example of the N-Type TFT, the functionality of the firstbarrier layer 142A depends on the relative difference of the maximumconduction band value of the adjacent layers, and thus, the material forforming the first barrier layer 142A can vary depending on the materialof the first oxide semiconductor layer 141A and the first gateinsulating layer 163A in contact with the first barrier layer 142A. Inway of an example, the first oxide semiconductor layer 141A may beformed of indium-gallium-zinc oxide (IGZO), and the first gateinsulating layer 163A may be formed of silicon nitride. In this example,the first barrier layer 142A may be formed of any one of copper oxide(Cu₂O), copper aluminum oxide (CuAlO₂), silicon oxide (SiO₂), strontiumcopper oxide (SrCu₂O₂), aluminum oxide (Al₂O₃) and a combinationthereof. For example, the first barrier layer 142A formed of Cu₂Oexhibits superior electron blocking ability, making it an excellentbarrier material for preventing the electron trapping in the first gateinsulating layer 163A of the P-type TFT.

The material for forming the first barrier layer 142A may depend onother factors in addition to the maximum valence band value/maximumconduction band value relationships discussed above. In both the N-typeand P-type TFTs, the interface between the surfaces of the first oxidesemiconductor layer 141A and the first gate insulating layer 163A mayhave defects, which allows electrons to be trapped therein during theoperation of the thin-film transistor. Reducing the defect density atthe interface of the first oxide semiconductor layer 141A and the firstgate insulating layer 163A can also increase the overall operationstability of the thin-film transistor. Accordingly, the material forforming the first barrier layer 142A may also depends on the defectdensity at the interface (which may also depend on the material of thefirst oxide semiconductor layer 141A and the first gate insulating layer163A). A material that exhibiting the interface defect density reductionfunctionality may be used for the first barrier layer 142A even if thematerial is not the best material for the hole blocking functionality.Further, the first barrier layer 142A may be formed of an alloy thatincludes a material exhibiting superior hole blocking functionality anda material exhibiting superior interface defect density reducingfunctionality. Also, the first barrier layer 142A may be formed of astack of layers, in which at least one of the layers is formed of amaterial for exhibiting the hole blocking functionality and at least oneof the layers is formed of a material for reducing the interface defectdensity of the adjacent layer (e.g., the first oxide semiconductor layer141A, the first gate insulating layer 163A).

Referring back to FIGS. 1A and 1B, the second active layer 150A as anupper active layer is formed on the first active layer 140A. The secondactive layer 150A is formed on the first active layer 140A to serve asthe upper active layer of the thin-film transistor 100A and provides aconductive channel at the upper side of the TFT (hereinafter referred toas “the upper channel”). The second active layer 150A includes a secondoxide semiconductor layer 151A that is formed on the first active layer140A to provide the upper channel and an intermediate barrier layer 152Aformed between the first oxide semiconductor layer 141A and the secondoxide semiconductor layer 151A to reduce the degradation of thethin-film transistor 100A by exposure to light. The second oxidesemiconductor layer 151A is overlapped with the second gate electrode122A is configured to be in contact with the source electrode 131A andthe drain electrode 132A, and to provide the upper channel when the gatevoltage is applied to the second gate electrode 122A. The second oxidesemiconductor layer 151A is formed on the intermediate barrier layer152A. The second oxide semiconductor layer 151A is formed to have thesubstantially same area as that of the intermediate barrier layer 152A.The second oxide semiconductor layer 151A may be formed of similar metaloxide material(s) as the first oxide semiconductor layer 141A describedabove.

The cross-sectional width “W2” of the second active layer 150A isnarrower than the cross-sectional width “W1” of the first active layer140A. The cross-sectional width of the active layer in the presentspecification means a length of the active layer from the sourceelectrode side end to the drain electrode side end of the active layer.The cross-sectional width “W2” of the second active layer 150A issmaller than the cross-sectional width “W1” of the first active layer140A such that some part of the first active layer 140A is not coveredby the second active layer 150A formed on the first active layer 140A.This allows the first active layer 140A to come in contact with thesource electrode 131A and the drain electrode 132A.

The intermediate barrier layer 152A is formed between the first activelayer 140A and the second oxide semiconductor layer 151A. Theintermediate barrier layer 152A is the layer for reducing thedegradation of the thin-film transistor 100A by light and serves as acharge (holes/electrons) trapping barrier for reducing degradation ofthe thin-film transistor 100A due to exposure to light. The material forforming the intermediate barrier layer 152 may be different depending onthe type of the TFT. For N-type TFT, the intermediate barrier layer 152Ais made from a material having the maximum valence band value smallerthan the maximum valence band values of the materials constituting thefirst oxide semiconductor layer 141A and the second oxide semiconductorlayer 151A. For P-type TFT, the intermediate barrier layer 152A is madefrom a material having the maximum conduction band value smaller thanthe maximum conduction band values of the materials constituting thefirst oxide semiconductor layer 141A and the second oxide semiconductorlayer 151A.

In the n-type thin-film transistor 100A, when the light illuminates thefirst oxide semiconductor layer 141A while the gate voltage is appliedto the first gate electrode 121A, holes may be generated in the firstoxide semiconductor layer 141A and the second oxide semiconductor layer151A, and the generated holes may move between the first oxidesemiconductor layer 141A and the second oxide semiconductor layer 151Ato be trapped in the first gate insulating layer 163A. Electrons may begenerated in the first oxide semiconductor layer 141A and the secondoxide semiconductor layer 151A by the number of the trapped holes, andthe threshold voltage of the thin-film transistor 100A may be shifted tocause the deterioration in characteristics of the thin-film transistor100A.

As previously explained, the intermediate barrier layer 152A made from amaterial having the maximum valence band value lower than the maximumvalence band values of the materials constituting the first oxidesemiconductor layer 141A and the second oxide semiconductor layer 151Amakes it difficult for the holes/electrons to move between the firstoxide semiconductor layer 141A and the second oxide semiconductor layer151A through the intermediate barrier layer 152A. More specifically, theintermediate barrier layer 152A serves as a barrier for preventing theholes/electrons in the second oxide semiconductor layer 151A to passthrough the intermediate barrier layer 152A and be trapped in the firstgate insulating layer 163A. Similarly, the intermediate barrier layer152A makes it difficult for the holes/electrons in the first oxidesemiconductor layer 141A to travel over to the second oxidesemiconductor layer 151A and be trapped in the second gate insulatinglayer 161A. As such, the intermediate barrier layer 152A can reduce thedeterioration in characteristics of the thin-film transistor 100A thatis likely to occur due to the shifted threshold voltage of the thin-filmtransistor 100A caused by the charge trapping phenomenon.

The material for forming the intermediate barrier layer 152A is selecteddepending on the materials forming the first oxide semiconductor layer141A and the second oxide semiconductor layer 151A. The material for theintermediate barrier layer 152A can be selected by considering thevalence band maximum value/conduction band maximum value relationshipwith the first and second oxide semiconductor layers 141A and 151A. Forexample, in cases of N-type TFT, the intermediate barrier layer 152A maybe made from various oxide materials such barium zirconate (BaZrO₃),zirconium dioxide (ZrO₂), magnesium oxide (MgO), gallium oxide (Ga₂O₃),strontium titanate oxide (SrTiO₃), tantalum oxide (TaOx), aluminum oxide(Al₂O₃), hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO₄), yttriumoxide (Y₂O₃), and titanium oxide (TiOx), and a combination thereof.Unlike the first barrier layer 142A discussed above, the interfacedefect density between the oxide semiconductor layers and the gateinsulating layers needs not be considered because the intermediatebarrier layer 152A is interposed between the first and second oxidesemiconductor layers 141A and 151A. As such, it is preferred that theintermediate barrier layer 152A is formed of TaOx, and more preferablyTa₂O₅. In case of a P-type TFT, the intermediate barrier layer 152A maybe formed of any one of copper oxide (Cu₂O), copper aluminum oxide(CuAlO₂), silicon oxide (SiO₂), strontium copper oxide (SrCu₂O₂),aluminum oxide (Al₂O₃) and a combination thereof.

The second gate insulating layer 161A is formed on the second activelayer 150A. The second gate insulating layer 161A insulates the secondactive layer 150A from the second gate electrode 122A. The second gateinsulating layer 161A may be the silicon oxide film, the silicon nitridefilm, or the stack of multiple layers including the silicon oxide filmand the silicon nitride film, but is not limited thereto. The secondgate insulating layer 161A may be made from various materials. Thesecond gate insulating layer 161A may be formed over the entire surfaceof the substrate 110A including the second active layer 150A. However,since the second gate insulating layer 161A has only to insulate thesecond active layer 150A from the gate electrode, the second gateinsulting film 161A may be formed only on the second active layer 150Aas shown in FIG. 1B.

The second gate electrode 122A as an upper gate electrode is formed onthe second gate insulating layer 161A. The second gate electrode 122Atransmits a driving signal to the thin-film transistor 100A. The secondgate electrode 122A is overlapped with the second active layer 150A,specifically, the second oxide semiconductor layer 151A of the secondactive layer 150A.

The second gate electrode 122A is made from a conductive material. Thesecond gate electrode 122A may be made from the same material as that ofthe first gate electrode 121A, but is not limited thereto. The secondgate electrode may be made from various materials. The first gateelectrode 121A and the second gate electrode 122A are electricallyconnected to each other. The second gate electrode 122A is formed abovethe first gate electrode 121A, and the first gate electrode 121A and thesecond gate electrode 122A may directly come in contact with each otheror may indirectly come in contact with each other through a separateconductive material. Accordingly, the same gate voltage is applied tothe first gate electrode 121A and the second gate electrode 122A.

The interlayer insulating film 162A is formed on the second gateelectrode 122A. The interlayer insulating film 162A may be made from thesame material as that of the second gate insulating layer 161A, but isnot limited thereto. The interlayer insulating film may be made fromvarious materials. In one embodiment, the interlayer insulating film162A may be formed over the entire surface of the substrate 110A, andmay be formed to have contact holes 172A for opening partial regions ofthe first active layer 140A and the second active layer 150A.

The source electrode 131A and the drain electrode 132A are formed on theinterlayer insulating film 162A. The source electrode 131A and the drainelectrode 132A are electrically connected to the first active layer 140Aand the second active layer 150A, respectively, through the contactholes 172A formed at the interlayer insulating film 162A and/or thesecond gate insulating layer 161A. The source electrode 131A and thedrain electrode 132A are made from conductive materials. The sourceelectrode 131A and the drain electrode 132A any one of, for example,molybdenum (Mo), aluminum (Al), chrome (Cr), gold (Au), titanium (Ti),nickel (Ni), neodymium (Nd) and copper (Cu), or an alloy thereof, but isnot limited thereto. The source electrode and the drain electrode may bemade from various materials. Moreover, the source electrode 131A and thedrain electrode 132A may be multiple layers made from any one selectedfrom a group consisting of molybdenum (Mo), aluminum (Al), chrome (Cr),gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd) and copper (Cu),or an alloy thereof.

The source electrode 131A and the drain electrode 132A come in contactwith at least one of a top portion and a side portion of the secondactive layer 150A and at least one of a top portion and a side portionof the first active layer 140A. As stated above, since the width of thesecond active layer 150A is narrower than the width of the first activelayer 140A, even though the second active layer 150A is formed on thefirst active layer 140A, the partial region of the first active layer140A is not covered by the second active layer 150A. Accordingly, asshown in FIG. 1B, the source electrode 131A and the drain electrode 132Acome in contact with the top portion of the first oxide semiconductorlayer 141A of the first active layer 140A and the top portion and theside portion of the second oxide semiconductor layer 151A of the secondactive layer 150A.

Some regions of the first oxide semiconductor layer 141A and/or thesecond oxide semiconductor layer 151A may be treated to increase theelectrical conductivity at the treated regions. When the electricalconductivity is increased at the partial region of the first oxidesemiconductor layer 141A and the partial region of the second oxidesemiconductor layer 151A, the corresponding regions have resistanceslower than that of the oxide semiconductor to which electricalconductivity has not been given. Accordingly, when the correspondingregions come in contact with the source electrode 131A and the drainelectrode 132A, contact resistances are also decreased. Therefore, thepartial region of the first oxide semiconductor layer 141A and thepartial region of the second oxide semiconductor layer 151A to which theelectrical conductivity is given may be a region of the first oxidesemiconductor layer 141A and a region of the second oxide semiconductorlayer 151A that come in contact with the source electrode 131A and thedrain electrode 132A.

The cross-sectional width of the first gate electrode 121A is equal toor greater than the cross-sectional width of the second active layer150A. A channel formed by the first gate electrode 121A corresponds to aregion of the first oxide semiconductor layer 141A overlapped with thefirst gate electrode 121A, and the source electrode 131A and the drainelectrode 132A that come in contact with the first oxide semiconductorlayer 141A may be formed to be closest to the region of the first oxidesemiconductor layer 141A overlapped with the first gate electrode 121A.In another exemplary embodiment, when the cross-sectional width of thefirst gate electrode 121A is narrower than the width of the secondactive layer 150A, electrical conductivity may be given to a region ofthe first oxide semiconductor layer 141A that is not overlapped with thefirst gate electrode 121A.

In the thin-film transistor 100A according to the exemplary embodimentof the present invention, the source electrode 131A and the drainelectrode 132A come in contact with the first oxide semiconductor layer141A of the first active layer 140A, and the first oxide semiconductorlayer 141A is overlapped with the first gate electrode 121A. Further,the source electrode 131A and the drain electrode 132A come in contactwith the second oxide semiconductor layer 151A of the second activelayer 150A, and the second oxide semiconductor layer 151A is overlappedwith the second gate electrode 122A. Furthermore, the first gateelectrode 121A and the second gate electrode 122A are electricallyconnected, and the same gate voltage is simultaneously applied thereto.Accordingly, when the gate voltage is applied to the first gateelectrode 121A and the second gate electrode 122A to turn on thethin-film transistor 100A, the upper channel is formed at the secondoxide semiconductor layer 151A of the second active layer 150A, and thelower channel is formed at the first oxide semiconductor layer 141A ofthe first active layer 140A. Thus, it is possible to provide thethin-film transistor 100A having a plurality of channels. Moreover,unlike a general structure of the thin-film transistor 100A in which thesource electrode 131A and the drain electrode 132A come in contact onlywith the top portion of the oxide semiconductor layer, since the sourceelectrode 131A and the drain electrode 132A come in contact with boththe top portion and along the thickness of the second oxidesemiconductor layer 151A of the second active layer 150A, an areabetween the source electrode 131A and the second oxide semiconductorlayer 151A and an area between the drain electrode 132A and the secondoxide semiconductor layer 151A increases. As a result, it is possible toenhance current flow of the thin-film transistor 100A to improve devicecharacteristics of the thin-film transistor 100A.

FIG. 1E is a cross-sectional view of a thin-film transistor according anembodiment of the present invention. Referring to FIG. 1E, a thin-filmtransistor 100E includes a substrate 110E, a first gate insulating layer163E, a first gate electrode 121E, a first active layer 140E, a secondactive layer 150E, a second gate insulating layer 161E, a second gateelectrode 122E, an interlayer insulating film 162E, a source electrode131E, and a drain electrode 132E. The substrate 110E, the first gateinsulating layer 163E, the first gate electrode 121E, the second activelayer 150E, the second gate insulating layer 161E, the second gateelectrode 122E and the interlayer insulating film 162E are substantiallythe same as the substrate 110A, the first gate insulating layer 163A,the first gate electrode 121A, the second active layer 150A, the secondgate insulating layer 161A, the second gate electrode 122A and theinterlayer insulating film 162A of FIG. 1B, and thus the redundantdescriptions thereof is omitted.

A width of the first active layer 140E is the same as that of the secondactive layer 150E, and the first active layer 140E is completelyoverlapped with the second active layer 150E. Since the width of thefirst active layer 140E and the width of the second active layer 150Eare the same and the first active layer 140E and the second active layer150E are completely overlapped with each other, the source electrode131E and the drain electrode 132E come in contact with a side portion ofa first oxide semiconductor layer 141E of the first active layer 140Eand a top portion and a side portion of a second oxide semiconductorlayer 151E of the second active layer 150E as shown in FIG. 1E.

FIG. 1F is a cross-sectional view of a thin-film transistor according toan embodiment of the present invention. Referring to FIG. 1F, athin-film transistor 100F includes a substrate 110F, a first gateinsulating layer 163F, a first gate electrode 121F, a first active layer140F, a second active layer 150F, a second gate insulating layer 161F, asecond gate electrode 122F, an interlayer insulating film 162F, a sourceelectrode 131F, and a drain electrode 132F. The substrate 110F, thefirst gate insulating layer 163F, the first gate electrode 121F, thesecond active layer 150F, the second gate insulating layer 161F, thesecond gate electrode 122F and the interlayer insulating film 162F arethe substantially same as the substrate 110A, the first gate insulatinglayer 163A, the first gate electrode 121A, the second active layer 150A,the second gate insulating layer 161A, the second gate electrode 122Aand the interlayer insulating film 162A of FIG. 1B, and thus theredundant descriptions thereof may not be presented.

A first oxide semiconductor layer 141F is formed on a first barrierlayer 142F. A width of the first barrier layer 142F is greater than awidth of the first oxide semiconductor layer 141F. For example, as shownin FIG. 1F, the width of the first oxide semiconductor layer 141F is thesubstantially same as that of the second active layer 150F formed on thefirst oxide semiconductor layer 141F, and the width of the first barrierlayer 142F is greater than the width of the first oxide semiconductorlayer 141F.

FIG. 1G is a cross-sectional view of a thin-film transistor according toan embodiment of the present invention. Referring to FIG. 1G, athin-film transistor 100G includes a substrate 110G, a first gateinsulating layer 163G, a first gate electrode 121G, a first active layer140G, a second active layer 150G, a second gate insulating layer 161G, asecond gate electrode 122G, an interlayer insulating film 162G, a sourceelectrode 131G, and a drain electrode 132G. The substrate 110G, thefirst gate insulating layer 163G, the first active layer 140G, thesecond active layer 150G, the second gate insulating layer 161G, thesecond gate electrode 122G, the interlayer insulating film 162G, thesource electrode 131G and the drain electrode 132G are the substantiallysame as the substrate 110A, the first gate insulating layer 163A, thefirst active layer 140A, the second active layer 150A, the second gateinsulating layer 161A, the second gate electrode 122A, the interlayerinsulating film 162A, the source electrode 131A and the drain electrode132A of FIG. 1B, and thus the redundant descriptions thereof may not bepresented.

The first gate electrode 121G functions as a light blocking layer. Thefirst gate electrode 121G is formed to prevent light entering from belowthe substrate 100G from reaching a second oxide semiconductor layer 151Gand a first oxide semiconductor layer 141G, and may be made from areflective conductive material. When light enters the first oxidesemiconductor layer 141G and the second oxide semiconductor layer 151Gwhile a bias is applied to the thin-film transistor 100G, reliability ofthe thin-film transistor 100G is affected. Thus, in the thin-filmtransistor 100G according to the embodiment of the present invention,since the first gate electrode 121G is made from the reflectiveconductive material, it is possible to prevent light entering from belowthe substrate 100G from reaching the first oxide semiconductor layer141G and the second oxide semiconductor layer 151G and to reducedegradation in reliability of the thin-film transistor 100G due toexposure to light. Further, in the thin-film transistor 100G accordingto the embodiment of the present invention, since a first barrier layer142G and a intermediate barrier layer 152G are used as layers foralleviating the degradation of the thin-film transistor 100G by light,the first gate electrode 121G needs not be formed as thick to shield theexternal light as in a case where the first barrier layer 142G and theintermediate barrier layer 152G are not used.

FIG. 1H is a cross-sectional view of a thin-film transistor according toan embodiment of the present invention. Referring to FIG. 1H, athin-film transistor 100H includes a substrate 110H, a first gateinsulating layer 163H, a first gate electrode 121H, a first active layer140H, a second active layer 150H, a second barrier layer 182H, a secondgate insulating layer 161H, a second gate electrode 122H, an interlayerinsulating film 162H, a source electrode 131H, and a drain electrode132H. The substrate 110H, the first gate insulating layer 163H, thefirst gate electrode 121H, the first active layer 140H, the secondactive layer 150H, the second gate insulating layer 161H, the secondgate electrode 122H, the interlayer insulating film 162H, the sourceelectrode 131H and the drain electrode 132H are the substantially sameas the substrate 110A, the first gate insulating layer 163A, the firstgate electrode 121A, the first active layer 140A, the second activelayer 150A, the second gate insulating layer 161A, the second gateelectrode 122A, the interlayer insulating film 162A, the sourceelectrode 131A and the drain electrode 132A of FIG. 1B, and thus theredundant descriptions thereof may not be presented.

The second barrier layer 182H is a layer for reducing degradation of thethin-film transistor 100H by light and serves as a charge trappingbarrier for reducing degradation of the thin-film transistor 100H due toexposure to light. The material for forming the second barrier layer182H depends on the type of the TFT (i.e., N-type or P-type) and may beselected from the materials described above in reference to the firstbarrier layer 142H. Similar to the first barrier layer 142H, thematerial(s) and the structure of the second barrier layer 182H may beselected by considering the interface defect density between the secondoxide semiconductor layer 151H and the second gate insulating layer161H.

The width of the second barrier layer 182H needs not be as long as thewidth of the second oxide semiconductor layer 151H. As shown in FIG. 1H,the width of the second barrier layer 182H can be equal to the width ofthe second gate insulating layer 161H. Referring to FIG. 1H, the widthof the second gate insulating layer 161H is the substantially same asthat of the second gate electrode 122H, a width of an upper channelformed at the second oxide semiconductor layer 151H is also thesubstantially same as that of the second gate electrode 122H.Accordingly, since the second barrier layer 182H covers a regioncorresponding to the width of the upper channel to thereby effectivelysuppress deterioration in characteristics of the thin-film transistor100H, the width of the second barrier layer 182H is the substantiallysame as that of the second gate insulating layer 161H.

FIG. 1I is a cross-sectional view of a thin-film transistor according toan embodiment of the present invention. Referring to FIG. 1I, athin-film transistor 100I includes a substrate 110I, a first gateinsulating layer 163I, a first gate electrode 121I, a first active layer140I, a second active layer 150I, a second gate insulating layer 161I, asecond gate electrode 122I, an interlayer insulating film 162I, a sourceelectrode 131I, and a drain electrode 132I. The substrate 110I, thefirst gate insulating layer 163I, the first gate electrode 121I, thesecond gate insulating layer 161I, the second gate electrode 122I, theinterlayer insulating film 162I, the source electrode 131I and the drainelectrode 132I are the substantially same as the substrate 110A, thefirst gate insulating layer 163A, the first gate electrode 121A, thesecond gate insulating layer 161A, the second gate electrode 122A, theinterlayer insulating film 162A, the source electrode 131A and the drainelectrode 132A of FIG. 1B, and thus the redundant descriptions thereofmay not be presented.

An intermediate barrier layer 142I is formed on a first oxidesemiconductor layer 141I, and a second barrier layer 152I is formed on asecond oxide semiconductor layer 151I. A width of the intermediatebarrier layer 142I is the same as that of the second oxide semiconductorlayer 151I, and a width of the second barrier layer 152I is the same asthat of the second gate insulating layer 161I.

FIG. 2A is a cross-sectional view of a thin-film transistor according toan embodiment of the present invention. Referring to FIG. 2A, athin-film transistor 200A includes a substrate 210A, a first gateinsulating layer 263A, a first gate electrode 221A, an active structure270A, a second gate insulating layer 261A, a second gate electrode 222A,an interlayer insulating film 262A, a source electrode 231A, and a drainelectrode 232A. The substrate 210A, the first gate insulating layer263A, the first gate electrode 221A, the second gate insulating layer261A, the second gate electrode 222A, the interlayer insulating film262A, the source electrode 231A and the drain electrode 232A are thesubstantially same as the substrate 110A, the first gate insulatinglayer 163A, the first gate electrode 121A, the second gate insulatinglayer 161A, the second gate electrode 122A, the interlayer insulatingfilm 162A, the source electrode 131A and the drain electrode 132A ofFIG. 1B, and thus the redundant descriptions thereof may not bepresented.

The active structure 270A is a structure for providing channels and isformed above the first gate electrode 221A. The active structure 270Aincludes a first oxide semiconductor layer 241A which is formed abovethe first gate electrode 221A and at which a lower channel is formed, asecond oxide semiconductor layer 251A that is formed above the firstoxide semiconductor layer 241A and at which an upper channel is formed,and an intermediate barrier layer 252A that is formed between the firstoxide semiconductor layer 241A and the second oxide semiconductor layer251A.

The first oxide semiconductor layer 241A is formed above the first gateelectrode 221A. The first oxide semiconductor layer 241A may be madefrom various metal materials.

The second oxide semiconductor layer 251A is formed on the intermediatebarrier layer 252A. A width of the second oxide semiconductor layer 251Amay be narrower than a width of the first oxide semiconductor layer241A. The second oxide semiconductor layer 251A may be made fromsubstantially the same material as that of the first oxide semiconductorlayer 241A.

The intermediate barrier layer 252A is formed between the first oxidesemiconductor layer 241A and the second oxide semiconductor layer 251A.The intermediate barrier layer 252A is a layer for insulating the firstoxide semiconductor layer 241A and the second oxide semiconductor layer251A from each other, which provide separate channels, respectively. Theintermediate barrier layer 252A is a layer for reducing degradation ofthe thin-film transistor 200A by light, and specifically, is a layer forreducing degradation of the second oxide semiconductor layer 251A formedon the intermediate barrier layer 252A and the first oxide semiconductorlayer 241A formed under the intermediate barrier layer 252A by light.The intermediate barrier layer 252A may be made from the same materialas that of the intermediate barrier layer 152A of FIG. 1B.

FIG. 2B is a cross-sectional view of a thin-film transistor according toan embodiment of the present invention. Referring to FIG. 2B, athin-film transistor 200B includes a substrate 210B, a first gateinsulating layer 263B, a first gate electrode 221B, an active structure270B, a second gate insulating layer 261B, a second gate electrode 222B,an interlayer insulating film 262B, a source electrode 231B, and a drainelectrode 232B. The substrate 210B, the first gate insulating layer263B, the first gate electrode 221B, the second gate insulating layer261B, the second gate electrode 222B, the interlayer insulating film262B, the source electrode 231B and the drain electrode 232B are thesubstantially same as the substrate 210A, the first gate insulatinglayer 263A, the first gate electrode 221A, the second gate insulatinglayer 261A, the second gate electrode 222A, the interlayer insulatingfilm 262A, the source electrode 231A and the drain electrode 232A ofFIG. 2A, and thus the redundant descriptions thereof may not bepresented.

A width of a first oxide semiconductor layer 241B of the activestructure 270B is the same as that of a second oxide semiconductor layer251B of the active structure 270B, and the first oxide semiconductorlayer 241B and the second oxide semiconductor layer 251B are completelyoverlapped with each other. The source electrode 231B and the drainelectrode 232B come in contact with at least one of a top portion and aside portion of the first oxide semiconductor layer 241B and at leastone of a top portion and a side portion of the second oxidesemiconductor layer 251B. Since the widths of the first oxidesemiconductor layer 241B and the second oxide semiconductor layer 251Bare the same and the first oxide semiconductor layer 241B and the secondoxide semiconductor layer 251B are completely overlapped, the sourceelectrode 231B and the drain electrode 232B come in contact with theside portion of the first oxide semiconductor layer 241B and the topportion and the side portion of the second oxide semiconductor layer251B as shown in FIG. 2B.

FIG. 3A is a conceptual diagram of a display device according to anexemplary embodiment of the present invention. FIG. 3B is an enlargedconceptual diagram of a sub-pixel region shown in FIG. 3A. A displaydevice 300 is a device for displaying an image and includes variousdisplay devices such as an organic light-emitting display (OLED) device,a liquid crystal display (LCD) device, or an electrophoretic display(EPD) device.

The display device 300 may be an organic light-emitting display device,and the organic light-emitting display device includes a substrate 310,a plurality of thin-film transistors, and an organic light-emittingdiode including an anode, an organic light-emitting layer and a cathode.The plurality of thin-film transistors for emitting the organiclight-emitting layer is included in a plurality of sub-pixel regions SPof the substrate 310 of the organic light-emitting display device 300.For example, as shown in FIGS. 3A and 3B, the plurality of thin-filmtransistors may include a switching transistor TR1 that transmits a datasignal from a data driving module 330 to a gate electrode of a drivingthin-film transistor TR2 when a scan signal is applied from a gatedriving module 320, and the driving transistor TR2 that transmitscurrent transmitted through a power supply module 350 to the anode inresponse to the data signal received from the switching transistor TR1and controls emitting of the organic light-emitting layer of thesub-pixel or the corresponding pixel by the current transmitted to theanode. Although not illustrated in FIGS. 3A and 3B, a thin-filmtransistor for a compensation circuit that prevents abnormal driving ofthe organic light-emitting display device may be further included. Theplurality of thin-film transistors of the organic light-emitting displaydevice may be one of the thin-film transistors according to the variousexemplary embodiments of the present invention.

When the organic light-emitting display device 300 is a transparentorganic light-emitting display device, each of the plurality ofsub-pixel regions SP of the organic light-emitting diode display device300 includes a light-emitting region and a light-transmitting region,and the thin-film transistor and the organic light-emitting diode may bearranged in the organic light-emitting region.

As mentioned above, in the thin-film transistors according to thevarious exemplary embodiments of the present invention, the first gateelectrode and the second gate electrode are electrically connected toeach other. A connection relation of the first gate electrode and thesecond gate electrode will be described in more detail with reference toFIGS. 3C and 3D.

FIG. 3C is a plan view for describing the display device according tothe exemplary embodiment of the present invention. FIG. 3D is across-sectional view taken along lines IIId-IIId′ and IIId″-IIId′″ ofFIG. 3C. Referring to FIGS. 3C and 3D, the display device 300 includes athin-film transistor including a substrate 310, a first gate insulatinglayer 363, a first gate electrode 321, a first active layer 340, asecond active layer 350, a second gate insulating layer 361, a secondgate electrode 322, an interlayer insulating film 362, a sourceelectrode 331 and a drain electrode 332, a gate wiring 323, a datawiring 333, and a pad 334. For the sake of convenience in description,FIG. 3D illustrates the same thin-film transistor as the thin-filmtransistor according to the embodiment of the present shown in FIG. 1B,but is not limited thereto.

In the thin-film transistors of the displace device 300 according to thevarious exemplary embodiments of the present invention, the first gateelectrode 321 and the second gate electrode 322 are electricallyconnected to each other. Referring to FIGS. 3C and 3D, the first gateelectrode 321 as a lower gate electrode is branched from the gate wiring323, and the second gate electrode 322 as an upper gate electrode iselectrically connected to the gate wiring 323. Specifically, at aportion where the second gate electrode 322 and the gate wiring 323 areelectrically connected (a portion of IIId″-IIId″″ in FIGS. 3C and 3D),the gate wiring 323 is formed on the substrate 310, the second gateelectrode 322 is formed on the second gate insulating layer 361 and thefirst gate insulating layer 363 formed on the gate wiring 323, and theinterlayer insulating film 362 is formed on the second gate electrode322. In order for the first gate electrode 321 and the second gateelectrode 322 to be electrically connected, contact holes are formed atthe interlayer insulating film 362 and the first gate insulating layer363 on the gate wiring 323 from which the first gate electrode 321 isbranched, contact holes are formed at the interlayer insulating film 362on the second gate electrode 322, and the pad 334 is formed on theinterlayer insulating film 362 at which the contact holes are formed toelectrically connect the gate wiring 323 and the second gate electrode322. In FIGS. 3C and 3D, it has been illustrated that the gate wiring323 and the second gate electrode 322 are electrically connected throughthe separate pad 334, the present invention is not limited thereto. Thegate wiring 323 and the second gate electrode 322 may be electricallyconnected in a direct contact manner. In FIGS. 3C and 3D, although ithas been illustrated that the second gate electrode 322 comes in contactwith the gate wiring 323 from which the first gate electrode 321 isbranched, a contact position of the first gate electrode 321 and thesecond gate electrode 322 may be variously changed depending on designs.For example, the second gate electrode 322 may be branched from the gatewiring 323, and the first gate electrode 321 and the second gateelectrode 322 may be electrically connected in such a manner that thefirst gate electrode 321 comes in contact with the gate wiring 323. Afirst gate wiring from which the first gate electrode 321 is branchedand a second gate wiring from which the second gate electrode 322 isbranched may be formed, and the first gate electrode 321 and the secondgate electrode 322 may be electrically connected in such a manner thatthe first gate wiring and the second gate wiring come in contact witheach other near a pixel region where the thin-film transistor ispositioned or at a non-display region of the substrate 310 distancedfrom the pixel region.

The gate wiring 323 is connected to the gate driving module 320 totransmit a gate voltage to the first gate electrode 321 and the secondgate electrode 322. That is, the gate wiring 323 may be directlyconnected to the gate driving module 320 to receive the gate voltagefrom the gate driving module 320, and the gate wiring 323 may beconnected to the first gate electrode 321 and the second gate electrode322 to apply the gate voltage thereto.

Referring again to FIGS. 3A and 3B, the display device 300 may be aliquid crystal display device, and the liquid crystal display deviceincludes a lower substrate, an upper substrate, a pixel electrode, acommon electrode, a color filter and a liquid crystal layer interposedbetween the upper substrate and the lower substrate. The liquid crystaldisplay device includes a plurality of pixel regions, and includes aplurality of thin-film transistors for individually driving theplurality of pixel regions. The plurality of thin-film transistors iselectrically connected to the pixel electrode formed on the lowersubstrate of each of the pixel regions to apply voltage to each pixelelectrode, liquid crystals are oriented by an electric field generatedbetween the pixel electrode positioned at the pixel region and thecommon electrode formed on the lower substrate or the upper substrate,and the oriented liquid crystals transmits selectively incident lightfrom a separate light source. In this way, the selectively transmittedlight passes through the color filter positioned on the upper substrate,so that an image is displayed. The plurality of thin-film transistors ofthe liquid crystal display device may be one of the thin-filmtransistors according to the various exemplary embodiments of thepresent invention.

The display device 300 may be an electrophoretic display device, and theelectrophoretic display device includes a lower substrate, an uppersubstrate, a pixel electrode, a common electrode and an optical mediumlayer. The optical medium layer is interposed between the uppersubstrate and the lower substrate, and includes a fluid and coloredcharged particles dispersed in the fluid. The electrophoretic displaydevice includes a plurality of pixel regions and a plurality ofthin-film transistors for individually driving the plurality of pixelregions. The plurality of thin-film transistors is electricallyconnected to the pixel electrode formed on the lower substrate of eachof the pixel regions to apply voltage to each pixel electrode and movesthe colored charged particles by an electric field generated between thepixel electrode positioned on the pixel region and the common electrodeformed on the upper substrate. The electrophoretic display device movesthe colored charged particles in the aforementioned manner, and a colorof the colored charged particles is displayed when the colored chargedparticles are positioned at a front surface of the electrophoreticdisplay device, for example, the upper substrate. The plurality ofthin-film transistors of the electrophoretic display device may be oneof the thin-film transistors according to the various exemplaryembodiments of the present invention.

When the thin-film transistors according to the various exemplaryembodiments of the present invention are used in the display device 300,a design of the thin-film transistor may be partially changed dependingon a kind of the display device 300. For example, when the displaydevice 300 is a flexible display device, since the display device 300needs to be repeatedly bent or folded, various elements constituting thethin-film transistor need to be easily bent or folded. Moreover, whenthe display device 300 is a transparent display device, even though thedisplay device 300 is viewed from one side, the other side of thedisplay device needs to be viewed to some extent. Accordingly, thevarious elements constituting the thin-film transistor may beconsiderably decreased in size, or may be made from transparentmaterials.

When the thin-film transistors according to the various exemplaryembodiments of the present invention are used in the display device 300,a design of the thin-film transistor may be partially changed dependingon an article in which the display device 300 is provided. For example,when the display device 300 is provided in a mobile device or asmall-sized device such as a cellular phone, a smart phone, a tablet PC,or a PDA, since a built-in battery is used without using an externalpower supply, the elements of the thin-film transistor may be designedto be suitable for the limited capacity of the battery. Further, whenthe display device 300 is provided in a fixation device or a large-sizeddevice such as a television, a monitor, a screen, or a billboard, sincea power is supplied from an external power supply, the elements of thethin-film transistor may be designed so as to realize higher definitionof the display device 300 due to a stabilized power supply.

When the thin-film transistors according to the various exemplaryembodiments of the present invention are used in the display device 300,a design of the thin-film transistor may be partially changed dependingon a place in which the display device 300 is provided. For example,when the display device 300 is provided at a high-humidity place such asa toilet, a basin, a shower room, or a kitchen, the thin-film transistormay be designed using moisture-resistance elements. Furthermore, whenthe display device 300 is provided at a place that is easily exposed toexternal impact, such as an external wall of a building, a window glassof a building or a window glass of a vehicle, the thin-film transistormay be designed using elements that easily absorb impact or have impactresistance.

The thin-film transistors according to the various exemplary embodimentsof the present invention are not limited to the aforementioned variousmodifications, and may be applied to various applications. The designsof thin-film transistors may be changed in various manners depending onthe applied applications.

FIG. 4 is a flowchart for describing a method for manufacturing athin-film transistor according to an exemplary embodiment of the presentinvention. FIGS. 5A to 5D are cross-sectional views of processes fordescribing the method for manufacturing a thin-film transistor accordingto the exemplary embodiment of the present invention.

First, a first gate electrode is formed (S40) on a substrate. Then afirst gate insulating layer is formed (S41) on the first gate electrode.The process of forming the first gate insulating layer is described inmore detail with reference to FIG. 5A.

Referring to FIG. 5A, a first gate electrode 521 is formed on asubstrate 510. The forming of the first gate electrode 521 may includeforming a metal material for a gate electrode 521 on the entire surfaceof the substrate 510 and selectively patterning the metal material for agate electrode 521 by, for example, a photolithography process.Subsequently, a first gate insulating layer 563 is formed on thesubstrate 510 on which the first gate electrode 521 is formed. The firstgate insulating layer 563 is formed to cover the first gate electrode521 on the substrate 510.

A first oxide semiconductor layer as a first active layer is formed(S42) on the first gate insulating layer, an intermediate barrier layeris formed on the first oxide semiconductor layer, and a second oxidesemiconductor layer is formed on the intermediate barrier layer.Processes of forming the first oxide semiconductor layer, theintermediate barrier layer and the second oxide semiconductor layer aredescribed in more detail with reference to FIG. 5B.

A first barrier layer is formed (S43). As described below, the firstbarrier layer may be formed on the first oxide semiconductor layer 541.However, the first barrier layer may be formed on the first gateinsulating layer 563.

In order to form a second oxide semiconductor layer 551 and anintermediate barrier layer (i.e., first barrier) 552, a material for asecond oxide semiconductor layer 551 and a material for an intermediatebarrier layer 552 may be formed on a first oxide semiconductor layer541, and the material for a second oxide semiconductor layer 551 and thematerial for a intermediate barrier layer 552 are patterned.

Subsequently, a second gate insulating layer is formed on the secondoxide semiconductor layer, and a second gate electrode is formed on thesecond gate insulating layer.

Referring to FIG. 5C, a second gate insulating layer 561 and a secondgate electrode 522 are formed on partial regions of the second oxidesemiconductor layer 551. The forming of the second gate insulating layer561 and the second gate electrode 522 may include forming a material fora second gate insulating layer 561 and a material for a second gateelectrode 522 on the entire surface of the substrate 510 and selectivelypatterning the material for a second gate insulating layer 561 and thematerial for a second gate electrode 522 by, for example, aphotolithography process.

Furthermore, although not illustrated in FIG. 5C, a second barrier layermay be formed on the second active layer 550. The second barrier layermay be made from the same that of the first barrier layer 542 or theintermediate barrier layer 552, and a width of the second barrier layermay be the substantially same as that of the second gate electrode 522.

In some exemplary embodiments, electrical conductivity may be given to apartial region of the first active layer 540 and a partial region of thesecond active layer 550. The giving of the electrical conductivity tothe partial region of the first active layer 540 and the partial regionof the second active layer 550 may include giving electricalconductivity to the partial region of the first active layer 540 and thepartial region of the second active layer 550 by using the second gateelectrode 522 and the second gate insulating layer 561 as masks. Thegiving of the electrical conductivity to the partial region of the firstactive layer 540 and the partial region of the second active layer 550is performed to reduce resistances of the partial region of the firstactive layer 540 and the partial region of the second active layer 550in contact with a source electrode 531 and a drain electrode 532.

Thereafter, the interlayer insulating film is formed on the second gateelectrode, the first oxide semiconductor layer and the second oxidesemiconductor layer, and a source electrode and a drain electrode thatare electrically connected to the first oxide semiconductor layer andthe second oxide semiconductor layer are formed. A process of formingthe interlayer insulating film and a process of forming the sourceelectrode and the drain electrode are explained in more detail withreference to FIG. 5D.

Referring to FIG. 5D, the forming of an interlayer insulating film 562may include forming contact holes for opening a partial region of thefirst active layer 540 and a partial region of the second active layer550 by forming a material for an interlayer insulating film 562 over theentire surface of the substrate 510 on which the gate electrode isformed and selectively patterning the material for an interlayerinsulating film 562 by a photolithograph process. After the interlayerinsulating film 562 including the contact holes is formed, the sourceelectrode 531 and the drain electrode 532 that are electricallyconnected to the first oxide semiconductor layer 541 and the secondoxide semiconductor layer 551 may be formed.

The exemplary embodiments of the present invention have been describedin more detail with reference to the accompanying drawings, but thepresent invention is not limited to the exemplary embodiments. It willbe apparent to those skilled in the art that various modifications canbe made without departing from the technical sprit of the invention.Accordingly, the exemplary embodiments disclosed in the presentinvention are used not to limit but to describe the technical spirit ofthe present invention, and the technical spirit of the present inventionis not limited to the exemplary embodiments. Therefore, the exemplaryembodiments described above are considered in all respects to beillustrative and not restrictive. The protection scope of the presentinvention must be interpreted by the appended claims and it should beinterpreted that all technical spirits within a scope equivalent theretoare included in the appended claims of the present invention.

What is claimed is:
 1. A thin-film transistor, comprising: a first gateelectrode; a first gate insulating layer disposed on the first gateelectrode; a first oxide semiconductor layer and a second oxidesemiconductor layer disposed on the first gate insulation film; anintermediate barrier layer separating the first and second oxidesemiconductor layers; a second gate insulating layer disposed on thesecond oxide semiconductor layer; a second gate electrode disposed onthe second gate insulating layer, the second gate electrode electricallyconnected to the first gate electrode; a source electrode electricallyconnected to the first and second oxide semiconductor layers; and adrain electrode electrically connected to the first and second oxidesemiconductor layers.
 2. The thin-film transistor of claim 1, wherein afirst current path is induced in the first oxide semiconductor layer byapplying a first voltage to the first gate electrode and a secondcurrent path is induced in the second oxide semiconductor layer byapplying a second voltage to the second gate electrode.
 3. The thin-filmtransistor of claim 2, wherein the first and second voltages areidentical.
 4. The thin-film transistor of claim 1, wherein theintermediate barrier layer has a lower maximum valance band value(V_(max)) than a maximum valance band value (V_(max)) of the first andsecond oxide semiconductor layers when the thin-film transistor is anN-type thin-film transistor, and the intermediate barrier layer has ahigher maximum conduction band value (C_(max)) than the first and secondoxide semiconductor layers when the thin-film transistor is a P-typethin-film transistor.
 5. The thin-film transistor of claim 1, whereinthe first oxide semiconductor layer has a cross-sectional width equal toor greater than that of the second oxide semiconductor layer, andwherein the source and drain electrodes are in direct contact with thefirst and second oxide semiconductor layers.
 6. The thin-film transistorof claim 1, further comprising at least one of: a first barrier layerinterposed between the first gate insulating layer and the first oxidesemiconductor layer; and a second barrier layer interposed between thesecond gate insulating layer and the second oxide semiconductor layer.7. The thin-film transistor of claim 6, wherein the first barrier layerhas a maximum valance band value (V_(max)) lower than that of the firstoxide semiconductor layer and the first gate insulating layer when thethin-film transistor is an N-type thin-film transistor, and wherein thefirst barrier layer has a maximum conduction band value (C_(max))greater than that of the first oxide semiconductor layer and the firstgate insulating layer when the thin-film transistor is a P-typethin-film transistor.
 8. The thin-film transistor of claim 6, whereinthe second barrier layer has a maximum valance band value (V_(max))lower than that of the second oxide semiconductor layer and the secondgate insulating layer when the thin-film transistor is an N-typethin-film transistor, and wherein the second barrier layer has a maximumconduction band value (C_(max)) greater than that of the second oxidesemiconductor layer and the second gate insulating layer when thethin-film transistor is a P-type thin-film transistor.
 9. The thin-filmtransistor of claim 1, wherein the first gate electrode is made of areflective conductive material.
 10. The thin-film transistor of claim 1,wherein, when the thin-film transistor is an N-type thin-filmtransistor, the intermediate barrier layer includes at least one of aTiOx, TaOx, SrTiO₃, BaZrO₃, ZrO₂, HfO₂, Al₂O₃, MgO, Ga₂O₃, and wherein,when the thin-film transistor is an P-type thin-film transistor, theintermediate barrier layer includes at least one of Cu₂O, CuAlO₂, SiO₂,SrCu₂O₂, Al₂O₃.
 11. A thin-film transistor, comprising: a first gateelectrode; an oxide semiconductor layer on the first gate electrode; afirst gate insulating layer interposed between the oxide semiconductorlayer and the first gate electrode; a source electrode and a drainelectrode that are electrically connected to the oxide semiconductorlayer; and a first barrier layer interposed between the oxidesemiconductor layer and the first gate insulating layer, wherein thefirst barrier layer has a maximum valance band value (V_(max)) lowerthan that of the oxide semiconductor layer and the first gate insulatinglayer when the thin-film transistor is an N-type thin-film transistor,and wherein the first barrier layer has a maximum conduction band value(C_(max)) greater than that of the oxide semiconductor layer and thefirst gate insulating layer when the thin-film transistor is a P-typethin-film transistor.
 12. The thin-film transistor of claim 11, whereinthe first barrier layer has a cross-sectional width equal to or greaterthan that of the oxide semiconductor layer.
 13. The thin-film transistorof claim 11, further comprising: a second gate insulating layer on theoxide semiconductor layer; and a second gate electrode on the secondgate insulating layer.
 14. The thin-film transistor of claim 13, furthercomprising: a second barrier layer interposed between the oxidesemiconductor layer and the second gate insulating layer, wherein thesecond barrier layer has a maximum valance band value (V_(max)) lowerthan that of the oxide semiconductor layer and the second gateinsulating layer when the thin-film transistor is an N-type thin-filmtransistor, and wherein the second barrier layer has a maximumconduction band value (C_(max)) greater than that of the oxidesemiconductor layer and the second gate insulating layer when thethin-film transistor is a P-type thin-film transistor.
 15. The thin-filmtransistor of claim 14, wherein, when the thin-film transistor is anN-type thin-film transistor, the first and second barrier layers includeat least one of a TiOx, TaOx, SrTiO₃, BaZrO₃, ZrO₂, HfO₂, Al₂O₃, MgO andGa₂O₃, and wherein, when the thin-film transistor is an P-type thin-filmtransistor, the first and second barrier layers include at least one ofCu₂O, CuAlO₂, SiO₂, SrCu₂O₂ and Al₂O₃.
 16. The thin-film transistor ofclaim 14, wherein the second barrier layer has a cross-sectional widthequal to or greater than that of the second gate insulating layer. 17.The thin-film transistor of claim 11, wherein the oxide semiconductorlayer includes indium (In), gallium (Ga) and Zinc (Zn).
 18. Thethin-film transistor of claim 11, wherein the first gate electrode ismade of a reflective conductive material.
 19. A method for manufacturinga thin-film transistor, the method comprising: forming a first gateelectrode on a substrate; forming a first gate insulating layer on thefirst gate electrode; forming a first oxide semiconductor layer on thefirst gate insulating layer; and forming a barrier layer comprising abarrier material having a maximum valance band value (V_(max)) lowerthan that of the first oxide semiconductor layer or a maximum conductionband value (C_(max)) greater than that of the first oxide semiconductorlayer.
 20. The method of claim 19, wherein the barrier layer is formedon the first gate insulating layer.
 21. The method of claim 20, furthercomprising: forming another barrier layer on the first oxidesemiconductor layer; forming a second oxide semiconductor layer on theother barrier layer; forming a second gate insulating layer on thesecond oxide semiconductor; and forming a second gate electrode on thesecond gate insulating layer.
 22. The method of claim 19, furthercomprising: forming a second oxide semiconductor layer on the barrierlayer; forming a second gate insulating layer on the second oxidesemiconductor; and forming a second gate electrode on the second gateinsulating layer.
 23. The method of claim 22, further comprising:forming another barrier layer between the second gate insulating layerand the second oxide semiconductor layer.